Conventionally, image display devices such as activematrix liquid crystal display devices have been widely used, as exemplified by liquid crystal display devices which employ thin-film transistors (TFTs) (TFT-LCD) as the pixel switching elements (“switching elements” hereinafter). In recent years, the liquid crystal display devices (LCD) have also been used in portable information terminals, portable phones, and the like.
The activematrix liquid crystal display device carries out display by a voltage modulation driving method in which, as shown in FIG. 59, a signal of a voltage according to image data is supplied to signal lines, and this voltage is then supplied to pixels which are selected by switching elements. Here, the switching elements are designed such that the voltage of the signal lines is sufficiently supplied to the pixel electrodes, i.e., a charging rate close to 100 percent (commonly, 99 percent or above) is attained. In this method, a required voltage is generated by an external circuit, and there is power consumption at a tone voltage generating section.
In display devices for which low power consumption is sought, such as portable information terminals and portable phones, this power loss adds up to a value which cannot be ignored. As a counter-measure, there has been proposed a method for carrying out tone display by supplying only an externally supplied reference voltage to the signal lines without the provision of the tone voltage generating section, and, as shown in FIG. 60, by controlling the charging rate according to a conduction period of the switching elements. Such a pulse width modulation driving method utilizing a two-value signal is disclosed, for example, in Japanese Unexamined Patent Publication No. 299388/1992 (Tokukaihei 4-299388) (published date. Oct. 22, 1992), Japanese Unexamined Patent Publication No. 140889/1980 (Tokukaisho 55-140889) (published date: Nov. 4, 1980), and Japanese Unexamined Patent Publication No. 62094/1991 (Tokukaihei 3-62094) (published date: Mar. 18, 1991).
The following describes the pulse width modulation driving (phase modulation driving). Unlike the driving method by voltage variance (voltage variance driving), the phase modulation driving employs modulation utilizing a pulse width to drive, for example, an activematrix liquid crystal display device which uses switching elements such as thin-film transistors (TFTs) or thin-film diodes. The switching elements have steep current-voltage characteristics and highly responsive, and thus accumulation of charge between the pixel electrodes and the counter electrode is rapid and the voltage between the electrodes increases at a high rate.
Therefore, the voltage applied between the pixel electrodes and the counter electrode varies according to a pulse width of a select voltage which was applied between a driving signal input terminal of the switching elements and the counter electrode. Thus, controlling the pulse width of the select voltage according to image data varies the applied voltage between the pixel electrodes and the counter electrode, thus controlling transmittance of pixels and carrying out tone display.
The following will explain the voltage variance driving and the phase modulation driving more specifically referring to drawings. FIG. 63 is a graph explaining a tone display mode by the voltage variance driving. As shown in FIG. 63, the voltage variance driving varies the level of an applied voltage to the liquid crystal according to image data so as to control transmittance of pixels and perform tone display.
This driving method by the voltage variance driving carries out tone display by varying the voltage value of a select voltage, and therefore requires a voltage signal as a driving signal in the same number as that of displayed tones. This necessitates a power circuit for outputting voltages of multi-levels as the number of displayed tones are increased, and the driving circuit is made complex as a result. Further, when the voltages of multi-levels are to be created from an input voltage, a step-up/step-down circuit, such as an operational amplifier, must be used to create pre-set voltages, which always accompanies a power loss. As a result, power consumption of the liquid crystal display device is increased.
The following will explain a tone display mode by the phase modulation driving. FIG. 64 is a graph explaining the tone display mode by the phase modulation driving. As shown in FIG. 64, the phase modulation driving carries out tone display by controlling the pulse width according to image data. That is, the power level applied to the liquid crystal is controlled by changing a pulse width, so as to perform tone display.
Unlike the voltage variance driving, the phase modulation driving employs the pulse width modulation mode, and thus allows a tone display only with voltages of two values without using the driving signal of multiple voltage levels as in the voltage variance driving. Performing tone display only with voltages of two values is very effective in reducing power consumption of the liquid crystal display device, because the voltage variance driving requires multiple voltage levels as described above. Further, creating pre-set voltages by the voltage variance driving results in power loss by the step-up/step-down circuit such as an operational amplifier.
On the other hand, in the phase modulation driving, the driving voltage in tone display only has two levels, and there is no power loss associated with step-up or step-down, thus driving the liquid crystal display panel at lower power consumption. Therefore, the liquid crystal display devices can be driven at lower power consumption with the phase modulation driving.
In practice, the pulse width modulation driving (phase modulation driving) is employed in liquid crystal display devices (MIM-LCD) which use an MIM element (metal-insulator-metal element), which is a two-terminal element, as the switching element. For example, Japanese Unexamined Patent Publication No. 326870/1999 (Tokukaihei 11-326870) (published date: Nov. 26, 1999) discloses a liquid crystal display device for portable information terminals, which employs the MIM element as the switching element. In the pulse width modulation driving method, a two-value voltage is outputted to the signal line, and there is no power consumption at the tone voltage generating section, and further, because a buffer is not required for each output with respect to the signal line, there is no constant current consumption at the tone voltage generating section and the buffer, thus having the advantage of lower power consumption over the voltage variance driving.
However, it is difficult by the foregoing conventional pulse width modulation driving to realize desirable multi-tone display while suppressing power consumption for the following reasons.
That is, as recited in the foregoing Tokukaihei 11-326870, it is not necessarily the case that a proportion of a conduction period of the switching element within one horizontal (1H) period should be set and allocated equally to each tone. This is explained in FIG. 61 and FIG. 62 which show a change in electrostatic capacitance. Here, FIG. 61 shows the case where a pixel is charged from 0 V to 5 V, and FIG. 62 shows the case where a pixel is charged from 0 V to −5 V.
The switching element is a thin-film transistor having a channel width and a channel length of 14 μm and 5 μm, respectively, and the pixel capacitance and the gate voltage are 0.5 pF and 10 V, respectively. As it can be expected from the standard equation of a delay circuit composed of a capacitor element and a resistance element, the voltage changes exponentially as a function of a charging time. Thus, a change in voltage of the pixel electrode is abrupt at the early stage and levels off (becomes gradual) as the voltage approaches the voltage of the signal line. The slope is about 0.5 V/μs in the vicinity of 2 V, which corresponds to a half-tone display of the liquid crystal display device, and if one is to have specifications capable of displaying 64 tones, controlling this would require a pulse width of about 60 ns. This is practically unachievable considering signal delays in wiring and non-uniform characteristics of the switching elements, and assuming that the signal line has a delay of, for example, 0.6 μs, the difference in slope between the input side and the non-input side of the signal line alone becomes equivalent of 10 tones. On the other hand, a change in voltage with respect to a charging time is small in the vicinity of the maximum level of charging which is required for a black display, and the allocated pulse width of one tone becomes about 12 μs at most, thus causing unbalance.
In order to actually realize the foregoing control, a very high frequency must be used for a reference clock which is used to generate a signal of a desired short pulse width within a signal line driver, and power consumption is increased as a result. That is, depending on the method of expressing tones, the frequency of the applied signal to the signal line is increased. Power consumption is generally proportional to frequency, and therefore, in the pulse width modulation driving method, the effect of lower power consumption is diminished as a whole by the increase in power consumption due to higher frequency, despite no power consumption at the tone voltage generating section and the buffer.
Further, the phase modulation driving has another problem that the display quality is easily changed by a change in ambient temperature of operation. One of the problems which is intrinsic to the liquid crystal display devices is that the display shows change with respect to ambient temperature of operation. This is likely to be caused by {circle around (1)} temperature characteristics (dielectric constant, retention, etc.) of a liquid crystal material, and {circle around (2)} temperature characteristics of the switching elements.
The behavior of a display change due to the liquid crystal material according to factor {circle around (1)} is basically the same in the voltage variance driving and the phase modulation driving. However, the behavior of the liquid crystal display device with respect to change in temperature characteristics of the switching elements according to factor {circle around (2)} differs greatly between the voltage variance driving and the phase modulation driving. The following will explain the reasons for this based on an example using the thin-film transistor (TFT) elements as the switching elements.
FIG. 65 is an equivalent circuit diagram per pixel of a liquid crystal display panel having the TFT elements. In the liquid crystal display panel having the TFT elements, the TFT elements are disposed at the intersections of the signal lines and the scanning lines, wherein the gate, source, and drain of a TFT element are connected to a scanning line, a signal line, and a liquid crystal capacitance, respectively. In this liquid crystal display panel, when the gate electrode becomes selected, the transistor is conducted and a video signal of the signal line is applied to the liquid crystal capacitance. When the gate electrode becomes non-selected, the transistor takes high impedance to prevent the video signal of the signal line from leaking into the liquid crystal capacitance.
FIG. 66 is a graph showing temperature dependance of Vg-√Id characteristics (Vg indicates a voltage applied to the gate electrode of the TFT element, and Id indicates a drain current) of a TFT (a-Si). It can be seen from the temperature characteristics in FIG. 66 that the drain current flown into the TFT increases with increase in temperature. The increased flow of the drain current means an increased current flow into the liquid crystal. This results in abrupt increase in drain voltage with respect to an input signal.
In view of the foregoing, the following considers the voltage variance driving and the phase modulation driving when there is a temperature change. First, the voltage variance driving is examined. FIG. 67(a) is a graph showing a tone signal (half-tone display) at temperature T=Tr (room temperature). In FIG. 67(a) the signal indicated by rectangular wave 1 is an input signal, and the signal indicated by curve 2 is a drain voltage. Here, it is assumed in the half-tone display that the set voltage Va is reached within a pre-set time period (application time: 1 H).
FIG. 67(b) is a graph showing a tone signal (half-tone display) when temperature T=Th (Th>Tr). FIG. 67(b) shows the case where T=Th by increasing the temperature from FIG. 55(a). It can be seen from FIG. 67(a) and FIG. 67(b) that the drain current flown into the TFT increases with increase in temperature and the drain voltage increases abruptly with respect to the input signal.
However, even though the drain voltage rises abruptly with increase in temperature, the change of this degree will not change the behavior of the voltage reaching the set voltage Va within a pre-set time period (application time: 1 H). As a result, the applied voltage to the pixel will not be changed by temperature, and there will be no change in tone display due to the temperature characteristics of the TFT. Evidently, however, the display does show a change in the voltage variance driving, when the characteristics of the TFT elements are changed by a larger temperature change.
The following considers the case of the phase modulation driving. FIG. 68(a) is a graph showing a tone signal (half-tone display) when temperature T=Tr. In FIG. 68(a), the signal indicated by a rectangular wave 1 is an input signal, and the signal indicated by a curve 2 is a drain voltage. Here, it is assumed in the half-tone display that the set voltage Vc is reached within a pre-set time period (application time: 1 H).
FIG. 68(b) is a graph showing a tone signal (half-tone display) when temperature T=Th (Th>Tr). FIG. 68(b) shows the case where T=Th by increasing the temperature from FIG. 68(a). The drain current flown into the TFT increases with increase in temperature, and the drain voltage with respect to the input signal increases abruptly. As a result, in response to this change in drain voltage, the set voltage Vc of the half-tone display is shifted higher than the case where T=Tr. As a result, when the temperature is increased, a voltage Vc′, which is increased by ΔV from a normal level, is applied, changing the tone display.
That is, the phase modulation driving employs the pulse width modulation mode, and thus the way a rise of the drain voltage is changed directly affects the tone display.
As a counter-measure for preventing display change due to a change in panel temperature in the liquid crystal display device, for example, Japanese Unexamined Patent Publication No. 10217/1991 (Tokukaihei 3-10217) (published date: Jan. 17, 1991) discloses a method of temperature compensation by changing a pulse width of a voltage applied to the signal electrodes according to temperature. However, the control in this conventional technique is very complex since it requires controlling a pulse signal according to tones.
Further, Japanese Unexamined Patent Publication No. 301094/1998 (Tokukaihei 10-301094) (published date: Nov. 13, 1998) discloses a method of preventing non-uniform image display in a transmissive liquid crystal display device by compensating for a change in threshold value of liquid crystal due to temperature distribution of a back light, according to a change in voltage of a scanning signal. However, this conventional technique only teaches compensating for a change in threshold value of liquid crystal in the transmissive liquid crystal display device, and is totally silent as to compensation with respect to the reflective liquid crystal display device, phase modulation driving, and switching element (TFT) characteristics.